Eggs are fairly easy to break open when force is exerted at a localized point, like cracking an egg open by hitting it against a bowl.

Yet when you squeeze an egg equally on the top and bottom, it is almost impossible to break it. A 2017 study (summarized here) examined how eggs can endure so much pressure.

So how can chicks break out of a shell that is so tough? It’s all about how the pressure is exerted.

Chicks break out of the shell through a series of localized points during the pipping process. The 2017 study found that chicken eggs can withhold 100 pounds of force distributed evenly on the top and bottom axes of the egg. Penguin chicks are able to use their beaks to push against the eggshell, and the shell breaks open because the chicks are exerting pressure at a localized point. As a penguin embryo develops, an air cell is formed at the larger end of the egg. When ready to hatch, the chick breaks through the membrane into the air cell and begins to use its lungs and the air within the cell for breathing. 

hand holding pipping egg
Penguin beginning to break out of its shell.

Following a brief rest, the chick is ready to start breaking out of the shell; it usually takes one to two days for the chick to fully hatch. The act of a chick breaking out of its shell is called pipping. Watch this!

African penguin pipping

How do these hardy shells help chicks?

Penguin parents incubate eggs anywhere from 32 to 66 days (depending on the species), and their bodies put evenly distributed pressure on the egg. The egg needs to be able to withstand this pressure and a bit of rolling around in case it gets out from under the parent, or when the parents switch off the egg to take turns incubating. Any cracking or breakage on the outside of the egg, which is a barrier to micro-organisms, could impair or end an embryo’s development and the possibility of hatching, so the strong egg shell is very important.

penguin in nest
A little blue penguin incubates its egg, covering it so well that we can barely see the egg underneath him!

Eggs at the Aquarium

Here at the Aquarium, we participate in the Species Survival Plan (SSP) for breeding to ensure that penguin populations in zoos and aquariums remain healthy and genetically diverse. Our breeding takes place behind the scenes so that we can monitor egg and chick development. But it’s an exciting day when the juvenile chicks graduate to the main exhibit!

penguin chicks and egg
Little blue penguin chicks with fake egg for size comparison.

If you’re 18 or older, you can even apply to volunteer or intern in the penguin exhibit. (Here’s a glimpse at what it’s like to be a penguin volunteer!)